Nonlinear analysis of a floating offshore wind turbine with internal resonances

In this work, a coupled 6-DOF model of the NREL 5-MW floating offshore wind turbine is proposed. The model takes into account gravitational, aerodynamic, hydrostatic, hydrodynamic and mooring loads. Wind and sea waves influence is studied through a nonlinear dynamic analysis for two types of platform configurations: OC3-Hywind spar buoy and MIT/NREL TLP. Possible internal resonances were detected by calculating the natural frequencies of the system, with a one-to-one resonance condition for the OC3-Hywind spar buoy, and a two-to-one resonance condition for the MIT/NREL TLP, for the heave, roll and pitch motions. The application of the method of multiple scales to the heave and pitch dynamics confirmed the existence of these resonances. The obtained frequency response curves were validated by the use of a direct numerical method, based on the finite difference method combined with a pseudo-arclength continuation technique. Cyclic-fold and Hopf bifurcations are detected when the MIT/NREL TLP platform is used. For certain excitation frequencies, Poincaré sections generated smooth densely filled curves indicating the presence of higher-order quasiperiodic motion for the heave and pitch displacements. Large jumps of the motion amplitude for the frequency and force responses indicate the existence of potential damage to the mooring system.